Oil is frequently used as an insulator in high voltage equipment, such as transformers. Over time, the insulating properties of the oil degenerate due to contamination by water, dust, or other products emanating from other materials within the transformer. It is known to test the insulating properties of this oil in an instrument called an oil test set. A sample of oil to be tested is contained within a test vessel within the oil test set. In such a test vessel, a steadily increasing voltage is applied across the sample of the oil until a spark flashes through the oil as a result of it breaking down electrically. The voltage at which this occurs is known as the breakdown voltage.
In some designs, the test vessel has a pair of electrodes mounted at the sides of the vessel in an adjustable manner so that a gap between the electrodes can be adjusted. In alternative designs, with the electrodes depending downwardly from a lid, for example, the electrodes are removed together with the lid when it is removed to replace the oil sample in the test vessel. Since the electrodes have been immersed in the oil sample, they are prone to dripping when removed.
Care must be taken in designing oil test vessels, because the breakdown voltage of air is considerably less than that of the oil under test. As a result, if there is insufficient insulation between the electrodes and the air surrounding the test vessel as compared to the gap between the electrodes in the oil, then the spark will not flash across the oil as desired, but will discharge through the atmosphere. This would lead to erroneous test results.